Polyvinyl alcohol (hereinafter, abbreviated as PVA)-based resin has an excellent gas-barrier property, toughness, transparency, and solvent resistance, and therefore is suitable for binder and wrapping material for various articles.
In general, PVA-based resin is substantially impossible to be melt-molded because the PVA-based resin has a melting point close to its decomposition temperature. For this reason, the preparation of aqueous solution of the PVA-based resin is required prior to molding PVA-based resin. The solution is cast and dried to provide a film, or the solution is applied to a surface of a variety of substrate and dried. Such molding processes inhibit wide use in PVA-based resin for wrapping.
In recent years, a PVA-based resin containing 1,2-diol structural unit at side chain thereof (hereinafter, sometimes called as “side chain 1,2-diol-containing PVA-based resin”) is suggested as a melt-moldable PVA-based resin having excellent gas-barrier property, in e.g. JP2004-075866A (patent document 1).
The side chain 1,2-diol-containing PVA-based resin is lowered in crystallinity due to steric hindrance of the side chain, comparing to conventional PVA-based resins, and has a lowered melting point. Nevertheless, the side chain 1,2-diol-containing PVA-based resin exhibits greater gas-barrier property than a conventional PVA-based resin. This is supposed that the side chain 1,2-diol-containing PVA-based resin would have an amorphous portion strongly hydrogen bonded between hydroxyl groups of the side chains, thereby securing excellent gas-barrier property regardless of the lowered crystallinity comparing to the conventional PVA-based resin.
High crystallinity and strong hydrogen bonds of the molecular chain of PVA-based resin give excellent gas-barrier property, but cause lowering of flexibility or impact resistance comparing to other thermoplastic resin.
It is widely made a study that a thermoplastic elastomer is added to PVA-based resin to form a sea-island structure in which PVA-based resin is sea part and thermoplastic elastomer is island part, thereby improving flexibility and impact resistance without impairing excellent gas-barrier property inherent in PVA-based resin.
For example, JP2001-114977A (patent document 2) discloses the improvement of flexibility and impact resistance of the PVA-based resin by adding a hydrogenated block copolymer obtained by hydrogenating a block copolymer containing at least two polymer blocks of vinyl aromatic compound as a thermoplastic elastomer and at least one polymer block of conjugated diene compound.
In order to obtain a PVA-based resin molded article having highly improved flexibility, a resin composition employing a combination suggested in patent document 2, namely a resin composition containing melt-moldable side chain 1,2-diol-containing PVA-based resin and thermoplastic elastomer (hydrogenated block copolymer), was studied and found that the flexibility was improved but not satisfied level. In particular, pinholes sometimes occurred in a film made from the resin composition when subjected to severe conditions like repeating flexion, which drastically lowers gas-barrier property of the film.
The present inventors have further studied a resin composition comprising the side chain 1,2-diol-containing PVA-based resin for improving bending fatigue resistance, and suggested, in JP2011-74364A (patent document 3), a resin composition employing a mixture of block copolymer without carboxyl group and block copolymer with carboxyl group as a thermoplastic elastomer component.